They already did poke fun at that kind of title in the game itself. Usually, 2 mooks will be talking about what's after Arkham City with this gem:

Thug 1: So what happens now?Thug 2: I guess we'll just stay here until they figure out what they're doing next.Thug 3: C'mon, man. What could they do next? Arkham County? Arkham Country? Big-ass Arkham World?Thug 2: I dunno, man. These guys are crazy, aren't they?Thug 3: Yes they are.

There's the big head mode hidden within the Cryptograph. There's also Scarecrow's hidden lab inside a boat.

The big head mode, they pretty much had to tell us. I'm sure there's more revealed soon....

It's also possible they add these things in the patch updates. XD

It makes you wonder since Calender Man talks about his murder victims on holidays, what if Rocksteady did a time release on certain villains but then again they probably want to release it as DLC instead.